A digital coherent transmission method, in which transmission of 100 (Gbps) or more is achieved by using a single wavelength light, is being developed, as demand of large capacity data transmission increases. In the digital coherent transmission method, in addition to optical intensity, an optical phase is also used in modulation of a signal, being different from intensity modulation method. The modulation method is such as PSK (Phase Shift Keying) or QAM (Quadrature Amplitude Modulation.
In the modulation method, a skew, a power difference and an orthogonal deviation (phase rotation) between an in-phase component signal and a quadrature phase component signal may occur in each of a transmitter and a receiver, because of various factors. The phenomena is called IQ distortion (alternatively, IQ unbalance or IQ imperfect) and may cause signal degradation. On other hand, there is disclosed a compensation means of IQ distortion caused in the receiver (for example, see Japanese Patent Application No. 2010-193204 and US Patent Application Publication No. 2011/0249981).